The entry of a protein into a mammalian cell is often dictated by a small segment of the protein, which is commonly referred to as a “protein transduction domain” or PTD. This segment can be used as a signal attached to a foreign protein to facilitate transport of such a protein into a mammalian cell. For example, amphipathic peptides are used to facilitate uptake of DNA-cleaving metalloporphyrins as potential antitumor drugs in human fibroblasts HS68 or murine lymphocytic leukemia L1210 cells (Chaloin, L. et al. Bioconjugate Chem. 12:691-700, (2001)). Peptides, called cell-penetrating peptides, such as penetratin, transportan, Tat (amino acids 47-57 or 48-60) and the model amphipathic peptide MAP, have been used as delivery vehicles for transporting pharmacologically important substances, such as antisense oligonucleotides, proteins and peptides (Hallbrink, M. et al. Biochim. Biophys. Acta 1515:101-109 (2001); Lindgren, M., et al. Trends Pharmacol. Sci. 21:99-103 (2000)).
Such peptides, particularly the DNA-binding homeodomain of Antennapedia, a Drosophila transcription factor, or the 21 residue peptide carrier Pep-1, are internalized by many types of cells in culture, such as human HS68 or murine NIH-3T3 fibroblasts, at either 37° C. or 4° C. The lack of effect of the temperature shift suggests a penetration mechanism different from that of classical endocytosis (Morris, M. C. et al. Nature Biotechnol. 19:1173-1176 (2001)), which requires chiral receptor proteins. One of the most widely used peptides to transport pharmacologically-active compounds in mammalian cells is the eleven amino acid arginine-rich protein transduction domain (PTD) of the human immunodeficiency virus type 1 (HIV-1) transactivator protein Tat (Schwarze, S. R. et al. Science 285:1569-1572 (1999), Schwarze, S. R. et al. Trends Cell Biol. 10:290-295 (2000)). Intraperitoneal injection of the 120 kDa beta-galactosidase/Tat fusion protein results in the transcellular transduction of the fusion protein into virtually all tissues in mice, including the passage of the blood-brain barrier. This short peptide domain of HIV-1 Tat has been shown to mediate cell internalization of large molecules or particles, including magnetic nanoparticles, phage vectors, liposomes and plasmid DNA. Unlike the other cell-penetrating peptides discussed above, internalization of cargo proteins by full length Tat or its 11 amino acid transduction domain is significantly impaired at 4° C. (Liu, Y. et al. Nat. Med. 6:1380-1387 (2000), Suzuki, T. et al. J. Biol. Chem. 277:2437-2443 (2002)) and requires interactions with receptors such as the heparan sulfate chains of the cell membrane heparan sulfate proteoglycans.
Most of the PTDs identified to date have been derived from viral and mammalian sources. Other sources of PTDs would be desirable for the design of various experimental sequences, and for animal and human therapies and prophylactic procedures. One alternative source of PTDs is bacterial cells. Although bacterial proteins such as cholera toxin are known to enter mammalian cell cytosol (Sofer, A. and Futerman, A. H. J. Biol. Chem. 270:12117-12122 (1995)), the cytotoxicity of such proteins has limited the use of bacterial proteins, or PTDs derived from them, for transporting pharmacologically important cargos in mammalian cells.